Non-volatile memory devices such as flash memory devices are used in many computers and electronic devices to store information. A flash memory device usually has a write operation to store information (e.g., data and instruction codes), a read operation to retrieve the stored information, and an erase operation to clear information from the memory. As demand for higher density memory device increases, three-dimensional (3D) memory devices have been proposed. An example of a conventional 3D memory device is described by Jiyoung Kim et al. in an article titled “Novel 3-D Structure for Ultra High Density Flash Memory with Vertical-Array-Transistor (VRAT) and Planarized Integration on the same Plane (PIPE)”, published in the 2008 Symposium on VLSI. Technology Digest of Technical Papers, pages 22-23. Since 3D memory devices are relatively new, manufacturing these devices can pose fabrication process challenges.